Me, I like variety, and what’s what determines for me what’s “best”.
Small lights like AAAs, it’s almost always twisty or nothing for me. Kinda retarded to have almost no grip on such a small light yet have to push a stiff tailclicky to operate it. Might as well save the length and make it a twisty.
Still small AA lights to me are best with a sideswitch, which I almost always hold pencil-style. Again, limited length, thus limited grip, it’s too cumbersome to have to crane your thumb into some unnatural position to turn it on/off. My Xeno E03 is like that, even with the finger-grooves in the side. The Immolate LD10, while short but not an AA light, is to me almost useless as a single-hander at all, because of the slick sides, complete and total lack of grip, and stiff tailswitch. You gotta use 2 hands: one to hold the light, and the other to press the switch.
2×AA and 2×AAA lights are great with tailclickies.
On the other extreme, big lights like an L2 (with 2 cells), L6, multicell Mags, etc., are cumbersome to use with a tailswitch, so should always have a sideswitch. My Tacklife and Cometa (single 26650, but big heads) are kinda on the edge of comfort as far as tailswitch lights, but still have great grip and aren’t too nose-heavy.
As for that middle ground, typische 18650 lights, I like all 3: tail, side, and dual. I almost always carry my lights icepick-style at eye- or neck-index (always have, even as a kid, even the big honkin’ gray 5D cell Rat Shack freebies), and whether side or tail, activate the light with either thumb (tail) or ring/pinky (side). No big deal, and in fact is second-nature to me. Only hold I don’t use, EVER, is “cigar-grip”. Sissy way to hold it, too flimsy, the light just wags around with the slightest hand/finger movement.
Dual-switch lights can be hit or miss, though. Zanflare F1, Nitefox UT20, etc., were the perfect size and grip to be able to click the tail with a thumb and change modes with the pinky, all with one secure grip. One rare exception is the SP31v2 which has the switches just too widely-spaced to be able to do that (well, unless you’re Andre The Giant, and he’s dead), and I gotta slide the light fore/aft as needed to work the switches.
One nasty aspect to almost all dual-switch lights is the f’n U-shaped cutouts around the tailswitch. Chances are pretty small that you’ll be able to bap the sideswitch comfortably without having 1 of the 4 corners of those cutouts on the tailcap digging into your thumb. Either leave the switch proud, or have it fully encircled.
So, as usual, there’s no One Perfect Way, and it all depends on the size/shape/length/weight of the light, how you prefer to hold it, what you’re using it for, how you’re using it (eg, constant vs momentary), and a gaggle of other factors. So yeah, I like variety. What I like, I buy. What I don’t like, I don’t buy. Same as anything else.